Thread Subject:

How to generate idmodel from acceleration data

Hi all,
       I have an acceleration data for which i need to find frf. i need to use 'arx' or any other function to make a parametric model( an 'idmodel') out of my data, which I will use in 'sim' function (along with input which i generated using 'idinput')to simulate output. I could find the frf using output and input . Plz clarify the syntax of arx function.
m = arx(data,orders).
m = arx(data,'na',na,'nb',nb,'nc',nc,'nk',nk)
my acceleration data has 19 outputs and 19 inputs(data is an iddata model). Plz tell what 'na' etc mean and what should be their values.
Is there any way other than arx to estimate the idmodel?
Thanks in advance.